1. Field of the Invention
This invention relates to a process for producing starch/polyolefin polymer compositions by polymerization of at least one olefin, preferably an aliphatic, low-molecular weight alpha-monoolefin, in an inert, liquid or gaseous or liquid and gaseous reaction medium at a temperature from -50.degree. to 145.degree. C. in the presence of a Ziegler-Natta catalyst obtained by treating dried starch with an aluminum trialkyl and a transition metal compound.
2. The Prior Art
Processes of this type are generally known, as disclosed in U.S. Pat. No. 3,704,271, and are aimed chiefly at producing polymer compositions with specific advantageous functional properties which are not obtained when the relevant mixtures are produced by physical methods. Such processes are also aimed at producing polyolefins of desired morphological properties. These are derived from said polymer compositions by at least partial separation of the starch. Polyolefins of this kind are designated in the literature as "nascent polyolefins."
Although the known processes, in principle, accomplish this object, they have not gained any practical importance. The reasons are essentially as follows.
The prior art almost exclusively recommends the use of Ziegler-Natta catalysts with transition metal halides, especially vanadium and titanium chlorides, as the transition metal compound. These are known to be highly corrosive and require the use of rather expensive non-corroding equipment.
Another disadvantage of the prior art is that the catalysts which are applied to starch frequently show a much lower activity and productivity than the Ziegler-Natta catalysts used in classical olefin polymerization. This requires the use of extremely large amounts of catalyst in industrial processes, causing catalyst cost to increase to an intolerably high level and raising the content of transition metal(s) in the crude products to a point where the polymer compositions either are no longer fit for quite a number of applications or require previous separation of transition metal(s) in a complicated and costly process.
It is therefore an object of this invention to improve the known processes of the type described above in such a way that the disadvantages involved in the state of the art are avoided or at least mitigated and especially to provide a process which can be carried out using equipment which does not require any special protection from corrosion and needs only comparatively small amounts of catalysts.